The present invention relates to a switch mode converter and to a deadband control therefor. The present invention also relates to a method of operating a converter.
The term "switch mode converter" is used herein in a broad sense, and encompasses DC to DC, AC to DC, AC to AC and DC to DC switch mode converters. The term "switch" is also used herein in a broad sense and can encompass two or more individual switch elements.
DC to DC switch mode converters convert a DC input voltage to a regulated DC output voltage by chopping the DC input voltage with a primary switch means to convert it to an AC voltage and then rectify and filter the AC voltage to provide the DC output voltage. A transformer or some other means may be provided to change the level of the chopped AC voltage so that the DC output voltage is obtained at the desired level after rectification and filtering. The DC output voltage is regulated by applying control signals to the primary switch means to vary its duty cycle and hence the duty cycle of the AC voltage obtained from chopping.
For a primary switch means including two or more primary switches coupled to the DC input voltage, the primary switches are selectively turned on and off so that the DC input voltage is conducted to an output of the primary switch means through the primary switches. For example, the primary switches can be coupled to the DC input voltage in a half bridge configuration (one pair of primary switches, the two switches of which are coupled in series across the DC input voltage), a full bridge configuration (two pairs of primary switches, the two switches of each pair being coupled in series across the DC input voltage) or a push/pull configuration (one pair of primary switches, the two switches of which are coupled in parallel with the DC input voltage through transformer windings). Each primary switch of a pair is alternately turned on and off by turn-on and turn-off control signals applied to the control inputs of the primary switch means so that each primary switch of a pair alternately conducts the input voltage to the primary switch means output. The converter duty cycle, i.e., the total on-time versus the overall switching period of the primary switches, is varied by the control signals to provide a constant DC output voltage as the DC input voltage and load conditions vary. However, it is imperative that both primary switches of a pair not be on simultaneously to avoid placing virtually a short circuit across the DC input voltage, a condition commonly referred to as "cross-conduction".
In order to prevent cross-conduction, typically, the converter cycle of operation includes a time period in which a signal is provided to the primary switch means control inputs to turn both primary switches of a pair off. This signal is provided between consecutive signals turning one and then the other of the primary switches of a pair on, and this time period is commonly referred to as a "deadband".
During normal operating conditions, the primary switches are controlled so as to preclude simultaneous closure of both primary switches of a pair. However, it is possible during certain operating conditions, such as when the input voltage to the converter is low and during certain dynamic load transients, for both primary switches of a pair to be closed simultaneously. In many converters, transistors are used as the primary switches. Since a transistor has storage time characteristics which delay turn off of the transistor, it is possible for a transistor to be on for a delay period even though a turn-off signal has been applied to the transistor. Cross-conduction can occur after a turn-off signal has been applied to one primary switch transistor of a pair and the next turn-on signal has been applied to the other primary switch transistor before the one primary switch transistor has actually turned off.
In certain instances, it is also possible for a primary switch to remain on for a period of time after a turn-off signal has been generated which is ineffective to turn the primary switch off due to other factors, such as transients generated in the drive circuitry for the primary switches which buck the turn-off signals.
Many switch mode converters utilize a fixed deadband which is calculated to provide sufficient time for the last primary switch of a pair that was on to turn off before a control signal is generated to turn the other primary switch of a pair on. In order to achieve safe operation over a variety of input and load conditions, the deadband period is made relatively long. While this ensures that cross-conduction does not ordinarily occur, it limits the maximum duty cycle obtainable from the converter and, hence, limits the overall conditions over which the converter can regulate. Ensuring against occurrence of cross-conduction by providing a fixed deadband, and seeking to obtain maximum duty cycle are competing considerations. It is desirable to make the deadband as short as possible and yet prevent cross-conduction.
U.S. Pat. No. 4,325,111 (Quioque) issued on Apr. 13, 1982, discloses an example of a switch mode converter in which a fixed deadband is provided.
U.S. Pat. No. 4,302,807 (Mentler) issued Nov. 24, 1981, discloses a switch mode converter having a primary switch turn-off circuit coupled to the converter output transformer which is polarity insensitive. This circuit insures that a turn-off signal is provided to the respective primary switch even if there is a momentary polarity reversal in the transformer secondary which would otherwise prevent the turn-off of one primary switch while the other primary switch is being turned on. However, there is no correlation between the turn-off signal and the actual state of the primary switches so that it is possible for a primary switch to remain on even after a turn-off signal has been supplied to that primary switch.
U.S. Pat. No. 4,061,930 (Nerem) issued Dec. 6, 1977, discloses a switch mode converter having a fixed deadband period obtained by counting a predetermined number of clock pulses when the base drive signal changes polarity state. In addition, a base drive signal is not provided to a primary switch transistor unless that primary switch transistor is forward biased as determined by a comparator coupled to the output of the transistor.
There is thus a need for a switch mode converter in which cross-conduction of the primary switches is prevented under all operating conditions of the converter without essentially sacrificing maximum duty cycle. The present invention provides such a converter.